In 1950, Pitler and Hurlich concluded that titanium showed promise as a structural armor against small-arms projectiles. Pitler et al.'s Some Mechanical and Ballistic Properties of Titanium and Titanium Alloys, Watertown Arsenal Laboratory (March, 1990). Titanium alloys are now being investigated for the same purpose. Ti--6Al--4V alloys, for example, have been used to form ballistic armor. See, for example, Hickey Jr. et al.'s Ballistic Damage Characteristics and Fracture Toughness of Laminated Aluminum 7049-773 and Titanium 6Al--4V Alloys, Watertown Arsenal Laboratory (March, 1980). The Ti--6Al--4V alloys comprise, as the name implies, titanium, 6 weight percent aluminum and 4 weight percent vanadium. Most of the Ti--6Al--4V alloys have relatively low oxygen concentrations of less than 0.20% by weight [all percents stated herein with respect to alloy compositions are percents relative to the total weight of the alloy, unless stated otherwise]. Ti--6Al--4V alloys having higher oxygen concentrations also are known, and such alloys have been used to produce ballistic plates. Love's U.S. Pat. No. 5,332,545, for example, describes ballistic plates made from a Ti--6Al--4V alloy. Love's alloy has a preferred composition of 6.2% aluminum, 4.0% vanadium and 0.25% oxygen.
Another titanium alloy that has been used to produce ballistic armor is discussed in J. C. Fanning's Terminal Ballistic Properties of TIMETAL.RTM. 62S, Titanium '95: Science And Technology (1996). Fanning describes a titanium alloy having 6.0% aluminum, 2.0% iron, a relatively low oxygen content of 0.18%, less than 0.1 weight percent vanadium and perhaps other trace elements. One measure of the effectiveness of ballistic plates is the average velocity (V.sub.50) of a shell, such as a 20 mm fragment-simulated projectile (FSP), required to penetrate such plates. Plates fashioned from Fanning's alloy were tested using the army's 20 mm FSP test. The V.sub.50 Fanning reported for such plates is 548 m/s. Id., Table III, page 1691. This V.sub.50 value is representative of most titanium alloys, which generally have V.sub.50 values for plates having thicknesses similar to Fanning's of less than 600 m/s.
The current military minimum V.sub.50 for a 0.625 inch (15.6 mm) thick plate made from Ti--6Al--4V ELI (extra low interstitial oxygen) using a 20 mm FSP test is 583 m/s. See military standard MIL-A-46077. For armor plates having a thickness of 16.1 mm to 16.9 mm, the V.sub.50 values currently required by the military range from 591 m/s to 612 m/s.
The Ti--6Al--4V alloys have been used to produce ballistic armor because they provide better ballistic results using less mass than steel or aluminum alloys against most ballistic threats. Titanium alloys are therefore referred to as being "more mass efficient" with respect to ballistic properties than steel or aluminum alloy. But, the V.sub.50 values of known titanium alloys are not entirely satisfactory, and such alloys are expensive to produce. As a result, there is a need for titanium alloys that can be formed less expensively than conventional titanium alloys, and which can be formed into ballistic plates having V.sub.50 values that meet or exceed current military standards.